The broadcaster and natural historian added that ‘the best we can hope is that we will slow it down and slow it down considerably’.

He made the comments during an interview with Channel 4 News.

Jon Snow told the legendary environmentalist an 11-year-old had written in to ask what the worst thing to happen in his life would be ‘if we don’t tackle climate change’.

Sir David replied: ‘I think civil unrest on a great scale, and mass migration on a great scale.

‘I think we will go on finding enough food, though it may not be precisely the choice that we would take freely.’

Asked if it was ‘too late to reverse climate change’, he said: ‘It’s too late to reverse it. Not only in my lifetime, but in the next lifetime.

‘I don’t think you can reverse it. I think the best that we can hope is that we will slow it down, and slow it down considerably.’

Sir David said it was ‘very difficult to detect real action in politicians’, adding that it was ‘really rather sad’ that the first climate debate among party leaders during an election only took place on Channel 4 last week.

He criticised Boris Johnson’s ‘shameful’ decision not to take part in the event, during which he was replaced with an ice sculpture, adding: ‘I don’t know what else he had to do, but it would have to be very, very important to dodge this one.’

When asked whether he thought the public would emerge from such a critical general election ‘with a better understanding of the need to change our ways’, Sir David said he doesn’t.

He said that the problem with short term elections was that politicians ‘think the issues facing the electorate are ones that are going to change their lives tomorrow or the day after, and those are the ones they concentrate on’.

Sir David said: ‘I don’t think things are going to get better. I think we can slow the degree to which they are getting worse.’